Currently we had a few TODOs concerned about unimplemented calls in
ANGLE. ANGLE now has support for ReadBuffer, so uncomment the calls.

BUG=angleproject:940

Change-Id: Ib297e3ab7f2fa2424ee2cc85918331066dae4475
Reviewed-on: https://chromium-review.googlesource.com/317340
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>